The present invention relates generally to travelling band screens, that is to say screens comprising a carrying frame and a screening band which is mounted so as to move as an endless chain in the said frame.
Screens of this type are used, for example, to equip water intakes, in which case they are usually arranged vertically in a sluice feeding the water intake.
Depending on their position relative to the direction of the flow of water to be screened, they can be classified in one or other of two large categories.
Firstly, there are the through-flow band screens that is to say band screens in which only the upstream face of the screening band receives the water to be screened, the generatrices of this screening band being approximately perpendicular to the direction of flow of the water.
Usually, the carrying frame of a through-flow band screen of this type is simply slid, under leaktight conditions, by means of engagement lips provided for this purpose, into grooves or guides arranged face to face in the two opposite walls of the sluice to be equipped.
However, these through-flow band screens have various disadvantages, which are as follows.
It is firstly necessary to provide a sealing plate, referred to as a boot plate, at the base of the screening band in order to prevent leakage between the latter and the bottom, of the pit and the baseplate must be tangential to the screening band.
Now, taking into account the unavoidable manufacturing and assembling tolerances and the wear of the components, in particular the components of the screening band carrying chains, a minimum clearance always remains between this baseplate and the screening band and, in practice, this clearance is never less than about 6 mm; it even frequently exceeds this value.
Consequently screening bands of these through-flow screens are not equipped with mesh having apertures smaller than 6 mm, and consequently the efficiency of the screening is limited thereby.
Furthermore, in these through-flow band screens, impurities and other debris, arrested on the upstream face of the screening band, which escape the back washing usually provided at the top part of the screen are reintroduced into the water, via the downstream face of the screening band, and carried downstream of the screen.
The second category of band screens is that of the dual-flow band screen, in which, with the screening band having its generatrices parallel to the direction of the flow water to be screened flows through both faces of the screening band, whether the water flows from the outside towards the inside of the screening band, or whether it flows in the opposite direction.
The carrying frame of these dual-flow band screens is usually built against masonry walls set up opposite one another in the sluice to be equipped.
Since no leaktightness is to be provided at the base of the screening band, these dual-flow band screens have the advantage of permitting the use, for screening band of meshes of reduced size, for example of less than 0.5 mm, compatible with the minimal sealing cap.
They also have the advantage of allowing the simultaneous use of both faces of screening band, and hence of doubling, for the same volume, the effective screening surface, that is to say the capacity of the apparatus.
Now, in the case of the installations currently equipped with through-flow band screens, there is the increasing problem of improving the efficiency of the screening by reducing the dimensions of the debris or organisms capable of being retained.
For the reasons explained above, through-flow band screens of this type cannot easily satisfy this requirement.
However, hitherto, their substitution by dual-flow band screens has not been easy to carry out, the physical conditions of insertion of the latter being different.
In general terms, the object of the present invention is, on the contrary, to facilitate this substitution when desired.